The present invention generally relates to an optical transmission apparatus and a control method thereof. More specifically, the present invention is directed to an optical transmission apparatus and a control method thereof, which are provided for an optical transmission system by which a plurality of signal light is wavelength-multiplexed so as to transmit the wavelength-multiplexed signal light.
In optical communication systems, generally speaking, while communication capacities are enlarged, in order to reduce costs of the optical communication systems, wavelength-multiplexed light transmission techniques have been applied. That is, in the wavelength-multiplexed light transmission techniques, a plurality of signal light having wavelengths different from each other are bundled within a single optical fiber so as to communicate the bundled signal light via the signal optical fiber. Also, in actual optical communication systems, in order to compensate losses as to power levels of optical signals occurred in an optical fiber which constitutes a transmission path between two points separated from each other over a distance, optical fiber amplifies are installed on the transmission path, and while the optical signals are not converted into electric signals during transmissions, the optical fiber amplifiers amplify the plurality of signal light having the different wavelengths from each other in a batch mode.
Optical fiber amplifiers have wavelength dependent characteristics in amplification factors (gains) with respect to signal light. For instance, in such a case of an optical fiber amplifier which amplifies optical signals having a wavelength range defined from 1530 nm up to 1560 nm, an amplification gain with respect to signal light having a wavelength near 1530 nm becomes higher than an amplification gain with respect to signal light having a wavelength near 1560 nm. This wavelength dependent characteristic of the gain changes in response to a change in gains of the optical fiber amplifier. In order to adjust the wavelength dependent characteristic of the gain of the optical fiber amplifier, it is desirable to realize that the wavelength dependent characteristic is not varied by keeping the gain constant. To this end, in optical fiber amplifiers, in order to achieve flat gain characteristics of signal light having a plurality of multiplexed wavelengths, a constant gain control capable of keeping a gain constant has been carried out. A constant gain control in an optical fiber amplifier may be realized by that for instance, while intensity of wavelength-multiplexed signal light on the input side of the optical fiber amplifier and intensity of wavelength-multiplexed signal light on the output side thereof are measured, pumping light of the optical fiber amplifier is controlled in such a manner that an intensity ratio (gain) of the input signal light to the output signal light always becomes constant.
Also, if a loss occurred in a transmission path changes only under the constant gain control, then intensity of an input optical signal of a signal repeater optical amplifier changes, and intensity of an output optical signal of the signal repeater optical amplifier changes in response to the first-mentioned intensity change. As a result, intensity of an input optical signal of a reception-sided optical transmission apparatus may eventually change, and thus, there are some possibilities that an input signal level of a receiver may be deviated from a dynamic range. In an optical transmission system, in order to solve the above-explained restriction in the input dynamic range of the receiver and also non-linear effects of optical fibers, a constant output intensity control (will be simply referred to as “constant output control” hereinafter) capable of keeping intensity of output signal light constant every wavelength is carried out in addition to the above-described constant gain control.
In a constant output control, for instance, total output light intensity of an optical amplifying unit containing an optical fiber amplifier is calculated based upon a total number of previously designated multiplexed signal light (namely, total multiplexed wavelength number) and output light intensity every wavelength; and an attenuating amount of an optical attenuating unit such as an optical attenuator provided at a post stage of the optical fiber amplifier may be controlled in such a manner that the total output light intensity of the optical amplifying unit becomes desirable light intensity. In accordance with this constant output control, the light signal outputted from the optical amplifying unit is controlled to have the desirable constant light intensity in such a manner that a loss change is canceled with respect to a variation in light intensity in combination with the loss change of the optical fiber.
As previously described, there are two control modes in the optical amplifying unit; namely, the constant gain control and the constant output intensity control capable of compensating the change in the signal intensity, which is caused by the loss variation of the transmission path. In this case, when a wavelength-multiplexed optical signal is optically amplified based upon the constant output intensity control, as previously described, since the total number of the multiplexed wavelengths is employed, a problem may occur in such a case that a difference is produced between the total multiplexed wavelength value, and a total wavelength number of multiplexed signal light which has been actually inputted to the optical fiber amplifier. In an optical transmission system, for example, in the case that a portion of plural sets of transmitters stored in optical fibers is brought into malfunction, and/or an optical fiber for coupling a transmitter to a wavelength multiplexing unit is extracted, a total number of signal light (total wavelength number) may change which is wavelength-multiplexed on the optical fiber. In this case, at a time instant when a failure happens to occur, the respective optical fiber amplifiers instantaneously cannot grasp a status of the actually multiplexed wavelength number. As a result, such a matching condition can be no longer established between a total wavelength number which constitutes an initial condition for the constant output intensity control, and a total number of wavelengths which have been physically multiplexed on the optical fiber.
As a consequence, when an optical signal having a certain wavelength drops due to a maintenance work, or a failure, while such a total output light intensity is employed as the target value, which is calculated based upon a larger signal light number than a total number of actually multiplexed signal light, a constant output intensity control of an optical fiber amplifier is performed. In this case, output light intensity per 1 signal light becomes higher than the predetermined intensity value. As a result, there is such a problem that each of the signal light reaches a receiver in an excessively high input signal level. As a consequence, when there is a change in a total number of multiplexed wavelengths, the execution of not the constant output intensity control, but the constant gain control is required. If the constant gain control is carried out, even when the total wavelength number changes, amplification gains of optical signals are constant, so that the optical signals having the respective wavelengths are amplified in the constant gain, and thus, excessively higher amplifications can be avoided.
As previously explained, the constant gain control must be selected with respect to the change in the total wavelength numbers, whereas the constant output intensity control must be selected with respect to the loss variation of the transmission path. As methods for switching these two control modes, for example, one technical idea “OUTPUT LEVEL CONTROL SYSTEM FOR WDM-PURPOSE OPTICAL AMPLIFIER” has been proposed in “Communication Society Meeting held by Electronic Information Telecommunication Institute in 1996”, Lecture No. B1096 lectured by YOSHIDA et. al. That is, in this output level control system, the intensity of the output light per 1 signal light is controlled to become the desirable intensity by detecting the total intensity of the signal light outputted from the optical amplifier, and the total number of the wavelengths stored in the optical transmission system. In this output level control system, the below-mentioned initial condition has been conducted. That is, under this initial condition, the speed as to the loss variation of the transmission path is sufficiently slow, as compared with the control speed of the optical amplifier, and on the other hand, the transient response characteristic of the signal intensity change in combination with the change in the total number of the wavelengths to be multiplexed is sufficiently fast, as compared with the control speed of the optical amplifier. Based upon the above-described initial condition, in the above-described output level control system, two sorts of the above-described variation factors may be discriminated from each other in accordance with a change in the changing speeds of the optical signal intensity measured in the optical transmission apparatus. The change in the wavelength numbers corresponds to, for example, such an event occurred in a changing operation of a communication path for connecting a transmission point to a reception point. Then, it is so supposed that a changing speed of optical signal intensity due to the above-described change in the wavelength numbers is lower than, or equal to several hundreds microseconds. The loss variation of the transmission path corresponds to, for example, such an abnormal event occurs when a maintenance worker of an optical transmission system pulls an optical fiber, or hooks an optical fiber. Then, it is so assumed that a changing speed of optical signal intensity due to the above-explained loss variation of the transmission path is higher than, or equal to several milliseconds.
When an attention is paid to the above-explained difference in the changing speeds, while a frequency threshold value has been previously set with respect to a total signal intensity change detected from multiplexed input light of the optical amplifier, the above-described control system can judge that the occurrence factor of the signal intensity variation is caused by either the change in the wavelength numbers or the loss change of the transmission path by checking whether or not a changing speed of total signal intensity in combination with an occurrence of a certain event exceeds the preset frequency threshold value. Also, in response to the judged occurrence factor of the total signal intensity change, such a control mode which should be performed in the optical amplifying unit can be determined. In this case, as the control mode of the optical amplifying unit, as previously explained, when the occurrence of the loss variation is detected, the constant output intensity control is employed, whereas when the change in the wavelength numbers is detected, the constant gain control is employed.
Also, as another method capable of solving the problems caused by the change in the wavelength numbers and the loss variation of the transmission path, JP-A-2001-257646 has proposed the below-mentioned control method: That is, the monitoring control-purpose monitoring light called as pilot light (probe light) is extracted by the branching element provided on the output side of the optical amplifier, and then, the optical amplifier is controlled in such a manner that the optical intensity of the probe light becomes constant. In the control method of JP-A-2001-257646, both the constant output intensity control and the constant gain control of the optical amplifier are carried out by paying an attention only to the intensity change of the probe light. As a result, in this control method, the factors as to the intensity changes of the signal light need not be discriminated from each other, whereas these factors are discriminated from each other in “Communication Society Meeting held by Electronic Information Telecommunication Institute in 1996”, Lecture NO. B1096 lectured by YOSHIDA et. al. Also, since there is also no specific restriction as to the responding time constant of the optical amplifier, the above-described optical amplifier control method can be applied with respect to such a high-speed loss variation (lower than, or equal to several milliseconds) of the transmission path, and further, such a low-speed change (higher than, or equal to several milliseconds) in the wavelength numbers.